Non volatile magnetic random access memory (MRAM) devices have the potential to replace volatile dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices in some applications. The MRAM devices typically comprise an array of memory cells such as tunnelling magneto-resistance (TMR), colossal magneto resistance (CMR), and giant magneto-resistance (GMR) memory cells.
In general, MRAM cells include a data layer and a reference layer. The data layer is composed of a magnetic material and in a write operation the magnetisation of the data layer can be switched between two opposing states by an applied magnetic field and thus binary information can be stored. The reference layer usually is composed of a magnetic material whose magnetization direction serves as a reference with respect to which the orientation of the data layer is measured. The reference layer could be either pinned, in which case its magnetisation is fixed due to an adjacent antiferromagnetic layer, or soft, in which case its magnetization is set dynamically.
For example, in a TMR cell, the data layer and the reference layer are separated by a thin dielectric layer which is arranged so that a tunnelling junction is formed. Any material has two types of electrons which have spin up and spin down polarities. In case of a magnetic layer that has a magnetisation, more electron spins have one orientation compared with the other one which gives rise to the magnetisation. The electrical resistance through the dielectric layer is dependent on the relative orientations of the magnetizations in the data and reference layers. This is the tunnelling magneto-resistance (TMR) effect and the state of the data layer can be determined by a sense current through the layers.
If the magnetic memory device includes an array of such magnetic memory cells, the individual magnetic memory cells are usually connected by column and row conductors. The purpose of these column and row conductors is two-fold. For switching the magnetization of a data layer in a particular magnetic memory cell which is located at the cross-point of a column and a row conductor, electrical currents are directed through the crossing row and column conductor and the associated magnetic field is used to switch magnetization of the data layer.
For reading the information stored in such a memory cell a sense current is directed through the crossing column and row conductors and through a selected magnetic memory cell. However, since a large number of magnetic memory cells are at the cross points of a large number of column and row conductors, the sense current is not exclusively directed through the magnetic memory cell that is to be read, but a portion of the sense current also penetrates parallel paths through adjacent memory cells. It is therefore difficult to select only a particular MRAM cell for a read-out operation.
The sense currents are very small currents and may be in the nA range. Therefore, the sense currents are difficult to measure and the difficulty to select only one magnetic memory cell in a readout process further complicates the measurement of a sense current. There is a need for an improved technical solution.